chore: import upstream snapshot with attribution

contributing/samples/multi_agent/task_sub_agent/README.md

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+# ADK Task as Sub-agent Sample
+
+## Overview
+
+This sample demonstrates how a "task mode" agent can act as a sub-agent to an LLM agent, effectively extracting structured data from a conversational flow.
+
+The main agent (`coordinator`) delegates interactions to two sub-agents:
+
+1. `order_collector`: A task agent that collects the user's food order (from a menu of Pizza, Burger, Salad) and returns a structured list of selected items as a `list[OrderItem]`.
+1. `payment_collector`: A task agent that collects the user's credit card and CVV information, returning a `PaymentInfo` object.
+
+Once the tasks are completed, the coordinator automatically uses a `place_order` tool with the structured data returned by both agents.
+
+## Sample Inputs
+
+- `I would like to order some food please.`
+
+- `I want 2 pizzas and 1 salad.`
+
+- `My credit card is 1234-5678-9012-3456 and my CVV is 123.`
+
+## Graph
+
+```mermaid
+graph TD
+    coordinator --> order_collector
+    coordinator --> payment_collector
+    coordinator -.->|uses| place_order[place_order tool]
+```
+
+## How To
+
+1. Define a sub-agent with `mode="task"` and an output schema:
+
+   ```python
+   order_collector = Agent(
+       name="order_collector",
+       mode="task",
+       output_schema=list[OrderItem],
+       ...
+   )
+   ```
+
+1. Assign it to a parent agent and use it in the instruction to collect the information:
+
+   ```python
+   coordinator = Agent(
+       sub_agents=[order_collector],
+       instruction="Delegate using `order_collector`...",
+       ...
+   )
+   ```
+
+## Related Guides
+
+- [LlmAgent Task Mode](../../../../docs/guides/agents/llm_agent/task.md) - Guide explaining the behavior and configuration of task-mode agents.